A spark plug for use in an internal combustion engine includes, for example, a center electrode extending in an axial direction; an insulator provided externally of the outer circumference of the center electrode; a cylindrical metallic shell externally assembled to the outer circumference of the insulator; and a ground electrode whose proximal end portion is joined to a forward end portion of the metallic shell. The ground electrode is bent at its substantially intermediate portion in such a manner that its distal end portion faces a forward end portion of the center electrode, thereby forming a spark discharge gap between the forward end portion of the center electrode and the distal end portion of the ground electrode.
In recent years, there has been proposed a technique for restraining an increase in the spark discharge gap resulting from spark discharges (for improving erosion resistance) while improving ignition performance, by means of joining a relatively-small-diameter tip formed of iridium, platinum, or the like to the forward end portion of the center electrode (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-68421). According to the technique, in a state in which the tip is placed on the forward end surface of the center electrode, a laser beam is radiated to the periphery of a contact surface between the tip and the center electrode so as to form a fusion zone where the tip and the center electrode are fused together, thereby joining the tip to the center electrode. The laser beam is radiated along a direction substantially parallel to the distal end surface of the tip, and, as a result of formation of the fusion zone, a portion of the tip located toward the side surface of the tip becomes smaller in thickness than a portion of the tip located toward the center of the tip.